The present invention relates to a printed circuit board and, in particular, to a printed circuit board which suppresses electromagnetic interference caused by interference of undesired electromagnetic waves.
As is well known, a digital electronic device comprises a number of electronic components, such as a random access memory (RAM), a read-only memory (ROM), a microprocessor, and the like each of which comprises a number of logic elements. These electronic components are mounted on a printed circuit board having signal lines (wired conductors) wired around thereon. In recent digital electronic devices, the operation speed of the logic elements has been rapidly increased and further the reduction in weight, thickness, length and size of the device has also been highly advanced. Following this, the mounting density of the electronic components onto the printed circuit board has also been rapidly enhanced.
However, since a signal flowing in the logic elements goes with abrupt changes in voltage and current, the electronic component is a noise generation source which generates high frequency noise. The high frequency noise causes interactions cooperatively with crosstalk noise or noise due to impedance mismatch so as to induce malfunctions relative to other electronic components on the printed circuit board and adversely affect other systems. Further, following the high-density mounting of the electronic components onto the printed circuit board and the reduction in size of the electronic components, electrostatic coupling between the electronic components and between the signal lines is increased so that the electromagnetic interference is liable to occur in the. digital electronic device.
Conventionally, in the general printed circuit board used in the digital electronic device, when operation frequency of signals flowing through the logic elements and the signal lines is low, electromagnetic coupling between the signal lines such as electromagnetic induction generated in the printed circuit board is relatively small so that no problem is raised. However, as the operation frequency of the signals flowing through the logic elements and the signal lines becomes high, the electromagnetic coupling between the signal lines is increased so that the foregoing problems are raised.
Therefore, there has been proposed an electromagnetic wave shield circuit board of a structure in which the printed circuit board itself is provided with a shield layer, wherein undesired electromagnetic waves generated at the printed circuit board are screened by this electromagnetic wave shield layer. To be concrete, the foregoing electromagnetic wave shield circuit board has a structure wherein two circuit board surfaces are opposite to each other with the shield layer interposed therebetween. Here, the shield layer is formed of a conductor material which reflects the electromagnetic waves. Specifically, the screening of the electromagnetic waves by means of the shield layer aims to prevent invasion of the electromagnetic waves further inward therefrom by reflecting the electromagnetic waves incident thereupon. Accordingly, in the electromagnetic wave shield circuit board, a shield effect can be expected with respect to one circuit board surface confronting the opposite circuit board surface on the side where an electronic component to be a noise generation source is installed, while, with respect to the opposite circuit board surface on the side where the electronic component to be the noise generation source is installed, undesired radiation is generated due to the reflection of the electromagnetic waves is generated. As a result, there have been not a few cases wherein secondary electromagnetic coupling is promoted at the opposite circuit board surface on the side of the noise generation source. Further, in a floating digital electronic device, since the shield layer works as an antenna, secondary radiant noise is possibly generated from the shield layer.
For solving such problems, in JP-A-8-46386 (herein-after referred to as the prior art), the applicant of this application has offered a printed circuit board which does not spoil a shield effect of an electromagnetic wave shield circuit board and thus has a sufficient shield effect against transmittance of the electromagnetic waves, while, with respect to reflection of the electromagnetic waves, does not promote electromagnetic coupling at least due to reflection. In the printed circuit board disclosed in this prior art, a printed circuit base member having wired conductors on one side or both sides thereof comprises a conductive support and insulating soft magnetic layers provided on both sides of the conductive support. The insulating soft magnetic layer includes soft magnetic powder and an organic binding agent. Further, the insulating soft magnetic layer may have a dielectric layer on at least one side thereof. This dielectric layer includes dielectric powder and an organic binding agent. Alternatively, the foregoing insulating soft magnetic layer may include the soft magnetic powder, the dielectric powder and the organic binding agent. It is preferable that the foregoing soft magnetic powder is powder being flat and/or needle-shaped.
On the other hand, a stacked body (electromagnetic interference suppressing layer) of the conductive support and the insulating soft magnetic body to be used in the foregoing printed circuit base member is produced, for example, in the following manner: Copper paste is formed into films on both sides of a polyimide film of 75 xcexcm by a doctor blade process to obtain a conductive support. Thereafter, soft magnetic paste is applied onto both sides of the conductive support by the doctor blade process to form insulating soft magnetic layers, which are then subjected to curing at 85xc2x0 C. for 24 hours to obtain a printed circuit base member.
Conventionally, a prepreg constituting a printed circuit board is composed of resin and glass cloth. For example, the conventional prepreg is produced by impregnating thermosetting resin into a glass cloth and then drying and half-curing it.
However, in the printed circuit board disclosed in the prior art, there is a problem that it is difficult to form the insulating soft magnetic layers. On the other hand, since the conventional prepreg is constituted of only the resin and the glass cloth, the radiant noise can not be absorbed.
Therefore, a theme of the present invention is to provide a magnetic prepreg which can absorb the radiant noise.
Another theme of the present invention is to provide a method of producing the foregoing magnetic prepreg.
Still another theme of the present invention is to provide a printed circuit board using the foregoing magnetic prepreg.
For achieving the foregoing themes, according to a first aspect of the present invention, there is obtained a magnetic prepreg formed by impregnating a magnetic paint composed of soft magnetic powder and thermosetting resin into a glass cloth. In the foregoing magnetic prepreg, it is preferable that the soft magnetic powder is metal powder being essentially flat. Further, it is preferable that a main component of the thermosetting resin is epoxy resin.
According to a second aspect of the present invention, there is obtained a method of producing a magnetic prepreg characterized in that a magnetic paint composed of soft magnetic powder and thermosetting resin is impregnated into a glass cloth, then dried and further cured into a half-cured state. In this method, it is preferable that a magnetic alignment procedure is carried out in an in-face orientation of the glass cloth after impregnating the magnetic paint into the glass cloth.
According to a third aspect of the present invention, there is obtained a printed circuit board using the foregoing magnetic prepreg, which comprises a normal prepreg composed of resin and glass cloth and at least one of the magnetic prepreg stacked to at least one side of the normal prepreg via a wiring pattern.